Osteoporosis is a bone disease characterized by low bone mass and loss of microarchitectural integrity, leading to increased bone fragility and risk of fracture. Treatments for this disease are only partially effective at increasing bone density and reducing fracture risk. Further work is needed to define the molecular signals controlling bone remodeling and determining how the myriad processes causing osteoporosis regulate these signals. There is evidence that sensory neuronal signaling can influence bone metabolism and that the loss of this signal might contribute to the development of osteoporosis. This proposal will use immunohistochemistry and real time PCR to identify sensory neurotransmitters and their receptors in osseal nerves and bone cells. Sensory transmitter modulation of bone cell functions will be determined in vitro. An in vivo approach will be to administer sensory neurotransmitter receptor antagonists systemically and by direct local infusion into the proximal tibia of rats to establish whether the endogenous release of these transmitters is required for the maintenance of bone integrity. Additional experiments will measure changes in bone neurotransmitter expression and content in osteoporotic rats, including a remote osteoporosis model in which the contralateral hindlimb loses bone mass, strength, and neuropeptide content after unilateral sensory lesioning or hindlimb immobilization. Neurotransmitter candidates that are down regulated in these osteoporosis models will then be directly infused into the tibias of osteoporotic rats in an attempt to reverse bone loss and increase bone strength. An additional line of investigation will be to determine whether chronic intrathecal NGF administration can restore neuropeptide levels in bone and reverse osteoporosis. It is anticipated that this project will; 1) identify sensory neurotransmitters capable of regulating bone cell proliferation, differentiation, and activity in vitro, 2) demonstrate that these transmitters and their receptors are present in bone, 3) verify that they are required for the preservation of skeletal integrity in vivo, 4) establish that these transmitters are depleted in bone after dorsal root gangliectomy and immobilization, and 5) demonstrate that restoration of these transmitters can reverse the local and remote osteoporotic effects of nerve trauma and immobilization. These investigations will help to identify the sensory transmitters that modulate bone remodeling and characterize their roles in osteoporotic processes.